VOLTAGE-DEPENDENT INTERACTION OF OPEN-CHANNEL BLOCKING MOLECULES WITHGATING OF NMDA RECEPTORS IN RAT CORTICAL-NEURONS

1. The mechanisms by which four adamantane derivatives (IEM-1857, -159 2, -1460 and -1754) block the open NMDA-activated channel were studied at membrane voltages (V-m) from -170 to +30 mV. The rate constants of channel block (k(+)) and of channel unblock (k(-)) were measured from the fully resolvable flicker of single-channel currents induced by ea ch compound. 2. The k(+) of each compound exhibited a similar exponent ial dependence on voltage over the V-m range studied. 3. The k(-) of I EM-1857 and IEM-1592 over the V-m range studied, and of IEM-1754 and I EM-1460 from -30 to -90 mV, exhibited similar exponential dependencies on voltage. However, the k(-) of IEM-1754 and IEM-1460 at V-m values more hyperpolarized than -90 mV mere much more steeply voltage depende nt, suggesting that at these V-m values the two drugs can occupy a dee per binding site. 4. Each of the drugs induced a concentration-depende nt prolongation of the mean burst length at -90 mV, suggesting that wh ile blocking they can interfere with channel closure. 5. The prolongat ion of mean burst length induced by the largest drug (IEM-1857) increa sed with hyperpolarization. The increase was consistent at each V-m wi th the predictions of the sequential scheme of block, suggesting that channel closure is prevented when IEM-1857 is bound. The prolongation of burst length induced by the smallest drug (IEM-1754) was less than predicted by the sequential scheme and the deviation increased with hy perpolarization. 6. The IEM-1857 concentration-dependence of number of blockages per unit open time had a slope equal to k(+) at -150 mV. Th e IEM-1754 concentration-dependence of number of blockages per unit op en time revealed a slope about two times less than k(+) for this compo und at -150 mV. 7. The mean patch current was not significantly altere d by 3 mu M IEM-1857 at V-m values from -90 to -150 mV, as expected of a drug that prevents channel closure when blocking. Mean patch curren t significantly decreased with hyperpolarization beyond -90 mV in the presence of 1 mu M IEM-1754. 8. The data suggest that there are two bl ocking sites at different depths within the NMDA-activated channel. Ch annel closure is prevented when any of the IEM drugs occupy the shallo w blocking site. Channel closure is permitted during occupation of a d eeper blocking site that can be reached only by the smaller IEM drugs at hyperpolarized voltages.